Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net income	$ 899	$ 859	$ 479
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net unrealized gains on investments with other-than-temporary impairments	(5)	3	(9)
Net unrealized gains on other investments	108	252	(552)
Net unrealized gains on investments	103	255	(561)
Foreign currency translation adjustment	100	(114)	(139)
Pension and postretirement benefits	2	1	(15)
Other comprehensive income (loss), net of tax	205	142	(715)
Total comprehensive income (loss)	$ 1,104	$ 1,001	$ (236)